1. Field of the Disclosure
Embodiments disclosed herein relate generally to a process for reducing the sulfur content of hydrocarbon streams. More specifically, embodiments disclosed herein relate to an improved process for the removal of sulfur compounds from a full boiling range naphtha, preferably fluid catalytically cracked naphtha. More particularly, embodiments disclosed herein relate to a process for the desulfurization of a fluid catalytically cracked naphtha wherein the valuable olefins are retained and recombinant mercaptans are prevented from forming, resulting in a low sulfur naphtha. More particularly, embodiments disclosed herein relate to a process which allows for more flexibility in varying the end point of the naphtha used in gasoline blending.
2. Background
Petroleum distillate streams contain a variety of organic chemical components.
Generally the streams are defined by their boiling ranges which determine the composition. The processing of the streams also affects the composition. For instance, products from either catalytic cracking or thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials and polyunsaturated materials (diolefins). Additionally, these components may be any of the various isomers of the compounds.
The composition of untreated naphtha as it comes from the crude still, or straight run naphtha, is primarily influenced by the crude source. Naphthas from paraffinic crude sources have more saturated straight chain or cyclic compounds. As a general rule most of the “sweet” (low sulfur) crudes and naphthas are paraffinic. The naphthenic crudes contain more unsaturates and cyclic and polycylic compounds. The higher sulfur content crudes tend to be naphthenic. Treatment of the different straight run naphthas may be slightly different depending upon their composition due to crude source.
Reformed naphtha or reformate generally requires no further treatment except perhaps distillation or solvent extraction for valuable aromatic product removal. Reformed naphthas have essentially no sulfur contaminants due to the severity of their pretreatment for the process and the process itself.
Cracked naphtha as it comes from the catalytic cracker has a relatively high octane number as a result of the olefinic and aromatic compounds contained therein. In some cases this fraction may contribute as much as half of the gasoline in the refinery pool together with a significant portion of the octane.
Catalytically cracked naphtha (gasoline boiling range material) currently forms a significant part (≈⅓) of the gasoline product pool in the United States and it provides the largest portion of the sulfur. The sulfur impurities may require removal, usually by hydrotreating, in order to comply with product specifications or to ensure compliance with environmental regulations. Some users require the sulfur of the final product to be below 50 wppm.
The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally copious quantities of hydrogen are included in the feed. The hydrodesulfurization reaction results in the production of hydrogen sulfide according to the following reaction: RSH+H2⇄R′+H2S. Typical operating conditions for standard single pass fixed bed HDS reactors, such as in a trickle bed reactor, are temperatures ranging from 600° F. to 780° F., pressures ranging from 600 to 3000 psig, hydrogen recycle rates ranging from 1500 to 3000 scf/bbl, and fresh hydrogen makeup ranging from 700 to 1000 scf/bbl.
After the hydrotreating is complete, the product may be fractionated or simply flashed to release the hydrogen sulfide and collect the now desulfurized naphtha. In addition to supplying high octane blending components the cracked naphthas are often used as sources of olefins in other processes such as etherifications, oligomerizations, and alkylations. The conditions of hydrotreating of the naphtha fraction to remove sulfur will also saturate some of the olefinic compounds in the fraction reducing the octane and causing a loss of source olefins. The loss of olefins by incidental hydrogenation is detrimental, reducing the octane rating of the naphtha and the reduction in the pool of olefins for other uses.
Various proposals have been made for removing sulfur while retaining the more desirable olefins. Since the olefins in the cracked naphtha are mainly in the low boiling fraction of these naphthas and the sulfur containing impurities tend to be concentrated in the high boiling fraction the most common solution has been prefractionation prior to hydrotreating. The prefractionation produces a light boiling range naphtha which boils in the range of C5 to about 250° F. and a heavy boiling range naphtha which boils in the range of from about 250-475° F.
The predominant light or lower boiling sulfur compounds are mercaptans while the heavier or higher boiling compounds are thiophenes and other heterocyclic compounds. The separation by fractionation alone will not remove the mercaptans. However, in the past the mercaptans have been removed by oxidative processes involving caustic washing. A combination oxidative removal of the mercaptans followed by fractionation and hydrotreating of the heavier fraction is disclosed in U.S. Pat. No. 5,320,742. In the oxidative removal of the mercaptans the mercaptans are converted to the corresponding disulfides.
Several U.S. patents describe the concurrent distillation and desulfurization of naphtha, including U.S. Pat. Nos. 5,597,476; 5,779,883; 6,083,378; 6,303,020; 6,416,658; 6,444,118; 6,495,030; 6,678,830 and 6,824,679. In each of these patents, the naphtha is split into two or three fractions based upon boiling point or boiling ranges.
U.S. Pat. No. 5,597,476 discloses a two-step process in which naphtha is fed to a first distillation column reactor which acts as a depentanizer or dehexanizer with the lighter material containing most of the olefins and mercaptans being boiled up into a first distillation reaction zone where the mercaptans are reacted with diolefins to form sulfides which are removed in the bottoms along with any higher boiling sulfur compounds. The bottoms are subjected to hydrodesulfurization in a second distillation column reactor where the sulfur compounds are converted to hydrogen sulfide and removed.
The use of two or more sequential catalyzed hydrodesulfurization treatments has been practiced for treating petroleum feeds to remove the organic sulfur compounds, however in these prior processes there is a significant quantity of “recombinant sulfur” in the product. Recombinant sulfur means organic sulfur compounds, mainly mercaptans, that are formed by the reverse reaction of hydrogen sulfide from the preceding hydrodesulfurization or hydrodesulfurizations in succeeding hydrodesulfurizations with olefins in the feed. The hydrogen sulfide can recombine to form mercaptans, as illustrated in the following reaction, thus increasing the amount of sulfur in the product.H2S+RC═CR′RC—CR′SH+R(SH)C—CR′.Also, the presence of hydrogen sulfide can cause more of the olefins to be saturated, losing octane and consuming hydrogen. The formation of mercaptans during the hydrodesulfurization of FCC gasoline is well known to occur, as disclosed in U.S. Pat. No. 2,793,170. Recombinant mercaptans may form due to the relatively high concentration of hydrogen sulfide in the flash or overhead system (compared to the concentration of hydrogen sulfide within a reactive distillation column, for example).
Accordingly, there exists a need for processes for the hydrodesulfurization of FCC gasoline which minimize or control the formation of recombinant mercaptans.